This invention relates to circuit analysis, and more particularly to evaluating the effect of interference noise generated by switching elements at points of interest in a circuit comprised of both digital and analog components.
Increasing numbers of circuit components are built on the same silicon substrate. Inevitably, some of the circuit components include sensitive analog and RF circuits, the performance which can be significantly degraded by the presence of interference signals. In a circuit comprised of both digital and analog components implemented on the same silicon substrate, the switching of the digital gates constitutes a significant source of interference noise, which propagate through, for example, the power supply grid, the silicon substratum signal wires and package pins.
Most of conventional analysis tools use traces from logic simulation to model the switching activity of digital gates. For example, a conventional method uses an inverter chain to model the digital switching activity. The digital signals are converted to time domain, and transient simulation is used to obtain the time-domain interference signal at points of interest. Another conventional approach has been to use logic simulation together with cell-library characterization. The individual contributions are combined at the output node into a long time-domain waveform. The corresponding power spectrum is estimated from this long time-domain waveform using standard techniques.
The conventional methods suffer from significant drawbacks. First, important features of the power spectrum are lost when spectral estimation is performed on time-domain sequences. Another disadvantage is the difficulty of incorporating nontrivial effects from the substrate and the power-ground grid. The substrate is assumed to be purely resistive, and the parasitics of the on-chip digital power-ground grid are usually ignored.
We have developed in accordance with the principles of the invention a method and apparatus for analyzing noise generated by switching elements in a circuit comprised of digital and analog components, wherein each switching element is associated with a random and discrete sequence of switching events having a plurality of possible values. Each one of the random and discrete sequences is modulated at each switching event with a set of continuous signals that represents the actual current injected by the corresponding switching element into the circuit. A spectral power density of the signals resulting from the modulation is transferred through a linear system that models the ensemble of power and ground wires, the substrate, the IC package, and the circuit board. The output of the linear system comprises the spectral power density of the interference signals at the points of interest.